Evaluating the effect of dietary soybean oil (S.O) on growth performances of juvenile ornamental fish Severum (Heros severus) was the main objective of this study. To achieve this objective, 462 fish were considered which were divided into 7 test groups including 6 groups of fishes (with an initial average weight of 0.6 ± 0.05 g and length of 2.3±0.35 cm) fed diets containing 6 levels of S.O (D1(3), D2(5), D3(8), D4(10), D5(12), D6(14)%) and the control group (CG) (receiving an oil-free diet) for 12 weeks with biometry intervals of 15 days. At the end of the test period, growth performance and nutritional parameters were measured. The study showed that the maximum weight was in fish fed diet containing 8% of S.O and also the maximum specific growth rate (SGR) (D3, p < /em><0.05) was obtained for this group. The highest rate of daily weight gain (DWG) was obtained for D3 which was significantly (p < /em><0.05) much more than that for D4, D6 and D5 (the lowest increase). The highest length gain (LG) was observed in D3 which was significantly more (p < /em><0.05) in comparison with the other treatments tested. The survival rate was over 80% in all test groups and showed no significant difference (p < /em>>0.05). At the end of the period, fish were evaluated for estimated analysis of the tissue. The results of this study which was focused on the growth and development of ornamental fish H. severus fed with different levels of soybean oil suggest that the optimal level of this type of oil in their diet is 8% based on the growth parameters.